The present invention relates to a device and a method for controlling the flow of electric power in a transmission line carrying alternating current. The invention also relates to an alternating current network comprising means for controlling the flow of electric power in the network.
In an electric transmission system it is of great importance and value to be able to rapidly and precisely control the flow of electric power so as to adapt the power flow to varying load conditions and to achieve a stable and predictable power flow despite disturbances of different kinds. Different types of devices have been proposed and put into operation for achieving this control of power flow. A device commonly used for this purpose is the so-called Unified Power Flow Controller (UPFC). The UPFC consists of two AC/DC voltage source converters designated as exciter and booster, respectively. The DC sides of both converters are connected to a common capacitor providing a DC voltage support for the converter operation and functioning as an energy storage means. The AC side of the booster inserts a synchronous AC voltage of controllable magnitude and phase angle in series with the transmission line via a series transformer. The AC side of the exciter is connected in parallel to the transmission line via a transformer where a current of controllable magnitude and power factor angle is injected into or absorbed from the transmission line. By means of a UPFC, the active and the reactive power flow through the transmission line can be controlled independently of each other. The main task of the exciter is to control the DC link voltage and to keep it on the reference value by exchanging the specific amount of active power with the transmission line. The secondary task of the exciter is to compensate reactive power as a var compensator so as to keep the line voltage on a constant level.
A UPFC constitutes an efficient device for controlling the flow of electric power in an electric transmission system. However, the series transformer of the UPFC has the full line voltage on both of its sides and must therefore have a high rating, which makes this transformer very costly and bulky.
The object of the present invention is to provide a device highly effective for controlling the flow of electric power in a transmission line carrying alternating current, which device can be manufactured at relatively low costs.
According to the invention, this object is achieved by a device comprising a first VSC (VSC=Voltage Source Converter) connected to the transmission line at a first point and a second VSC connected to the transmission line at a second point, said first and second VSCs having their DC sides connected to a common capacitor unit, wherein the device further comprises a by-pass switch connected to the transmission line between said first point and said second point in parallel with the first and second VSCs so that the first and second VSCs will operate as a back-to-back station when the by-pass switch is open and as two parallel static var compensators when the by-pass switch is closed.
When the by-pass switch of the inventional device is open and the VSCs operate as a back-to-back station, a powerful control of the flow of electric power in the transmission line can be achieved. During this first mode of operation, the phase, the frequency as well as the magnitude of the alternating voltage in the transmission line can be controlled by means of the device, and the active and reactive power can be controlled independently of each other. When the by-pass switch of the inventional device is closed the VSCs operate as two parallel static var compensators. During this second mode of operation, the device indirectly controls the transmission line voltage, and thereby the transmitted electric power, by generating reactive power for, or absorbing reactive power from, the transmission system. The device is preferably operated in said first mode during time periods when the flow of electric power in the transmission line has to be controlled to a large extent, whereas the device is preferably switched over to said second mode during time periods when only minor regulations or no regulations at all of the power flow are required. When the device is operated in said second mode, the power losses are lower than during operation in said first mode.
The device according to the invention constitutes a very flexible and efficient means for controlling the flow of electric power in a transmission line, which device does not require any costly and bulky transformers for its operation.
As appears from above, the by-pass switch of the device is used for switching between the two above-mentioned modes of operation. Furthermore, the by-pass switch constitutes a safety appliance for the VSCs of the device in that the by-pass switch can be arranged to close when the current in the transmission line becomes so strong that it could damage the components, such as the semiconductor elements, of the VSCs. When the by-pass switch is closed, the current by-passes the back-to-back station constituted by the two VSCs. Consequently, the VSCs do not have to have a rating determined by the maximum line current and can therefore be manufactured at a relatively low cost.
A further advantage with the device according to the invention is that it can be used in a transmission line where there might occur a temporary variance in the frequency of the alternating current on the generating side of the transmission line. When operating in said first mode, i.e. when the VSCs are operating as a back-to-back station, the VSCs of the device can namely be controlled to achieve a stable frequency of the current in the transmission line on one side of the device despite a variance in the frequency of the current in the transmission line on the other side of the device. This cannot be done with a UPFC.
According to a preferred embodiment of the invention, the VSCs comprise semiconductor elements of turn-off type constituted by bipolar transistors with an insulated gate (IGBT=Insulated Gate Bipolar Transistor). It is particularly advantageous to use IGBTs as semiconductor elements of turn-off type in this context, since they may easily be controlled simultaneously, so that they will behave as one single semiconductor element of turn-off type or breaker.
According to another preferred embodiment of the invention, the VSCs are arranged to conduct the current through the device in such a way during the breaking operation of the by-pass switch that essentially no current or only a limited current passes the switch, so as to assist the switch in said breaking operation. In this way, the by-pass switch can have a relatively low rating and can therefore be manufactured at a relatively low cost. Another advantage of the ability to use a by-pass switch having a relatively low rating is that a switch having a low rating can perform the switching operation more rapidly than a switch having a higher rating.
The invention also relates to a method for controlling the flow of electric power in a transmission line carrying alternating current having the features of claim 11, as well as an alternating current network having the features of claim 12.
Further advantages as well as advantageous features of the invention will appear from the following description and the dependent claims.